R. G. Carson & S. Riek

Perception and Motor Systems Laboratory, Department of Human
Movement Studies, The University of Queensland, Brisbane, Australia

Muscle activation patterns are determined, in part, by the mechanical context
in which movements are executed. In bilateral movements of the forearm (pronation-supination),
the position of the axis of rotation of the lower arm has a profound influence
upon the stability of interlimb coordination. These findings suggest that the
nature of the coupling between the limbs is strongly determined by musculo-skeletal
constraints. In the present study we sought to investigate one facet of this
coupling: the modulation of corticospinal input to upper limb motoneurones,
that results from the voluntary movement of the contralateral limb. Five participants
performed rhythmic pronation-supination movements of the left forearm in time
with an auditory metronome. In separate blocks of trials, the external axis
of rotation was located either above or below the long axis of the forearm.
Surface electromyographic (emg) recordings were obtained from the flexor carpi
radialis (FCR), extensor carpi radialis (ECR), pronator teres (PT), and biceps
brachii (BB) muscles of both arms. Motor potentials were evoked in the (quiescent)
muscles of the right arm by transcranial magnetic stimulation of the left motor
cortex. We calculated the degree of association (correlation coefficients) between
the r.m.s. emg amplitudes recorded from the muscles of the moving limb, and
the amplitudes of the motor evoked potentials obtained from homologous muscles.
It was evident that the excitability of the contralateral motor pathways was
influenced by the mechanical context in which the movements were performed.
When the axis of rotation was above the limb, correlation coefficients for FCR
(F(1, 4) = 12.47, p<0.05, f = 0.79) and PT (F(1, 4)
= 7.76, p<0.05, f = 0.46) were lower than when the axis of rotation
was below the limb. In contrast, the correlation coefficients for BB were higher
when the axis of rotation was above the limb, than when the axis was below the
limb (F(1, 4) = 6.41, p=0.06, f = 0.47). The correlation coefficients
obtained for ECR were not influenced by the mechanical context (F(1,
4) < 1, p>0.20, f = 0.1).